With the progression of the multimedia era, various types of liquid crystal display devices, from a small one used in a projector device, a cellular phone, a viewfinder, and the like to a large one used in a notebook PC, a monitor, a television, and the like, have rapidly become widespread. A medium-sized liquid crystal display device has become essential in electronic equipment such as a viewer and a PDAs (Personal Digital Assistants) and in amusement devices such as portable game machines and pachinko (Japanese pinball game) machines. These liquid crystal display devices find use in a variety of locations even in such appliances as refrigerators and microwave ovens.
At present, almost all liquid crystal display elements are of the twisted nematic (referred to as “TN” below) display type. A liquid crystal display element of the TN display type utilizes a nematic liquid crystal substance. If a conventional TN cell is subjected to direct matrix drive, display quality is not very high and the number of scan lines is limited. Accordingly, if direct matrix drive is adopted, use is made mainly of liquid crystal of the STN (Super Twisted Nematic) type, rather than of the TN type. Liquid crystal of this type exhibits improved contrast and viewing angle dependence in comparison with early direct matrix drive employing TN-type liquid crystal. Since the speed of response is low, however, this approach is not suited to display of moving pictures.
In order to improve upon the display performance afforded by direct matrix drive, an active matrix scheme in which each pixel is provided with a switching element has been developed and is now in wide use. By way of example, TN-TFT-type liquid crystal generally is used. Such a liquid crystal cell employs a thin-film transistor (TFT) in a TN-type display scheme. Since an active matrix scheme using a TFT provides a display quality higher than that obtained with direct matrix drive, TN-TFT liquid crystal presently dominates the market.
Owing to demand for ever-higher image quality, methods that provide an improved viewing angle have undergone research and development and some of these methods are in practical use. As a result, high-performance liquid crystal displays that are primarily in use at the present time are TFT-type active matrix liquid crystal displays of the following three types:                displays that employ a compensating film in a TN-type cell;        in-plane-switching (IPS) mode displays; and        multidomain vertically aligned (MVA) mode displays.        
In order to perform positive and negative write using a 30-Hz image signal in these active matrix liquid crystal display devices, rewriting is carried out every 60 Hz and the duration of one field is about 16.7 ms (the total time of both positive and negative fields is referred to as one frame and is about 33.3 ms).
By contrast, the response speed of liquid crystal at the present time is on the order of this frame period even in a fastest condition when one considers the response during display of halftones. This means that a response speed faster than the present frame period is necessary when an image signal comprising a moving picture is to be displayed, when high-speed computerized images (computer graphics) are displayed, and when a high-speed game image is displayed.
On the other hand, mainstream pixel size at present is on the order of 100 ppi (pixels per inch), and higher definition is contemplated using the following two methods:
The first method is to raise machining precision to reduce pixel size.
The second method is to employ a field-sequential (time-division) color liquid crystal display device in which backlighting for illuminating the liquid crystal display is changed over to red, green and blue in time-wise fashion and red, green and blue images are displayed in sync with this changeover. This approach makes possible a three-fold increase in definition over the prior art since it is unnecessary to spatially dispose color filters.
With a field-sequential liquid crystal display device, it is necessary to display single color for one-third time of the single field, and therefore the time available for display is about 5 ms. Accordingly, it is required that the liquid crystal itself have a response faster than 5 ms.
Owing to the necessity for such high-speed liquid crystal, a various technologies have been studied and several high-speed display mode technologies have been developed. These high-speed liquid crystal technologies can be classified generally into two major trends.
The first consists of technologies for raising the speed of the above-mentioned nematic liquid crystal most widely in use.
The second consists of technologies using spontaneous-polarization smectic liquid crystal that exhibits spontaneous polarization and can respond at high speed.
The speedup of nematic liquid crystal, which is that in widest use, is mainly carried out by the following means:
(A) reducing the cell gap and increasing electric field intensity at the same voltage;
(B) applying a high voltage to raise field intensity and facilitating a change in state (this is an overdrive method);
(C) lowering viscosity; and
(D) using a mode considered to be a high-speed mode in principle.
The following problems arise even with such nematic liquid crystal of elevated speed:
Since response of liquid crystal ends substantially in one frame in the case of a high-speed nematic liquid crystal, there is very large change in the capacitance of the liquid crystal layer ascribable to anisotropy of the dielectric constant. Owing to the change in capacitance, a change occurs in holding voltage to be written to and retained in the liquid crystal layer. This change in holding voltage, i.e., a change in the effective applied voltage, results in insufficient writing and therefore lowers contrast.
Further, if the same signal continues to be written, luminance continues changing until the holding voltage no longer changes and several frames become necessary in order to obtain stable luminance.
In order to prevent such a response that necessitates several frames, it is necessary that one-to-one correspondence be established between the applied signal voltage and the transmittance obtained.
With active matrix drive, transmittance after the liquid crystal responds is decided not by the applied signal voltage but by the amount of electric charge that has accumulated in the liquid crystal capacitor after the liquid crystal responds. The reason for this is that active matrix drive is constant-charge drive that causes the liquid crystal to respond by the electric charge held.
If minute leakage and the like are ignored, the amount of electric charge supplied from an active element is decided by accumulated charge that prevailed prior to predetermined signal write, and newly written charge.
Further, accumulated charge after the liquid crystal has responded varies depending upon the physical constants of the liquid crystal, the electrical parameters thereof and pixel design values such as accumulation capacity. In order to establish one-to-one correspondence between applied signal voltage and transmittance, therefore, the following are required:
(A) correspondence between signal voltage and write charge;
(B) accumulated charge prior to write; and
(C) information for performing calculation of accumulated charge after response, as well as the actual calculation.
The above necessitates a frame memory for storing (B) over the entire screen and a calculation unit for (A) or (C).
A reset pulse method of applying a reset voltage to bring liquid crystal to a predetermined liquid crystal state is one method of establishing one-to-one correspondence between applied signal voltage and obtained transmittance without using the above-mentioned frame memory and calculation unit, and this method is often employed. An example of this method is described in the prior art set forth in H. Nakamura, K. Miwa and K. Sueoka, “Modified drive method for OCB LCD”, 1997 IDRC (International Display Research Conference), SID L-66-L-69 (Non-Patent Document 1). According to this reference, use is made of an OCB (Optical Compensated Birefringence) mode in which orientation of nematic liquid crystal is made a pi-shaped orientation and a compensating film is applied.
Response speed of this liquid crystal mode is approximately 2 to 5 ms, which is much faster than the conventional TN mode. As a result, response should end in one frame. As mentioned above, however a large-scale decline in holding voltage occurs owing to a change in dielectric constant ascribable to the response of the liquid crystal, and several frames are needed to obtain stable transmittance.
A method of writing a black image without fail following the writing of a white image in one frame is indicated in FIG. 5 of Non-Patent Document 1. The diagram of FIG. 5 of this reference is cited as FIG. 13 in the drawings accompanying this application. In FIG. 13, time is plotted along the horizontal axis and luminance along the vertical axis. The dashed line in FIG. 13 indicates a change in luminance in the case of ordinary drive. A stable luminance is reached is the third frame.
In accordance with the reset pulse method, a predetermined state is always obtained when new data is written and therefore one-to-one correspondence between a written constant signal voltage and constant transmittance. Owing to this one-to-one correspondence, the generation of a driving signal becomes very simple and means such as a frame memory for storing the previously written information becomes unnecessary.
The structure of a pixel in a liquid crystal display device of active matrix type will now be described.
FIG. 10 illustrates an example of a pixel circuit for one pixel in a conventional liquid crystal display device of active matrix type. As shown in FIG. 10, the pixel of the liquid crystal display device comprises a MOS transistor Qn (referred to simply as “transistor Qn” below) having its gate electrode connected to a scan line (or scanning signal electrode) 901, either its source electrode or drain electrode connected to a signal line (or image signal electrode) 902, and the other of these source and drain electrodes connected to a pixel electrode 903; a storage capacitor 906 formed between the pixel electrode 903 and a storage capacitor electrode 905; and liquid crystal 908 sandwiched between the pixel electrode 903 and an opposing electrode (or common electrode) Vcom 907.
In notebook personal computers that constitute a large part of the market for liquid crystal displays, an amorphous silicon thin-film transistor (referred to as an “a-Si TFT below) or polycrystalline silicon thin-film transistor (referred to as a “p-Si TFT”) usually is used as the transistor (Qn) 904, and NT liquid crystal is employed as the liquid crystal material.
FIG. 11 illustrates an equivalent circuit of a TN liquid crystal cell. As shown in FIG. 11, the equivalent circuit of a TN liquid crystal cell is expressed by a circuit in which a capacitor component C3 (electrostatic capacitance Cpix thereof) of the liquid crystal is connected in parallel with a resistance value Rr of a resistor R1 and a capacitor C1 (electrostatic capacitance Cr thereof). In this equivalent circuit, the resistance value Rr and electrostatic capacitance Cr are components that decide the response time constant of the liquid crystal.
FIG. 12 illustrates a timing chart of scan line voltage Vg, signal line voltage (or image signal voltage) Vd and voltage Vpix of the pixel electrode 903 (referred to as “pixel voltage” below) in a case where the above-mentioned TN liquid crystal is driven by the pixel circuit shown in FIG. 10.
As shown in FIG. 12, the scan line voltage Vg attains a high level VgH during the horizontal scanning period. As a result, the transistor (Qn) 904 is in the ON state during this period and the signal line voltage Vd being input to signal line 902 is transferred to the pixel electrode 903 through the transistor (Qn) 904. The TN liquid crystal normally operates in a mode in which light passes through when no voltage is applied. This is a so-called “normally white mode”.
In the example shown in FIG. 12, the voltage for increasing optical transmittance through the TN liquid crystal is applied across several fields as the signal line voltage Vd. When the horizontal scanning period ends and the scan line voltage Vg reverts to the low level, the transistor (Qn) 904 reverts to the OFF state and the signal line voltage that has been transferred to the pixel electrode 903 is held by the storage capacitor 906 and capacitance Cpix of the liquid crystal. The pixel voltage Vpix at this time gives rise to a voltage shift, which is referred to as a “field-through voltage”, via the gate-source capacitance of the transistor (Qn) 904 at the moment the transistor (Qn) 904 attains the OFF state.
This voltage shift is indicated at Vf1, Vf2 and Vf3 in FIG. 12. The amount of the voltage shifts Vf1 to Vf3 can be reduced by designing the storage capacitor 906 to have a large value.
In the next field period, the pixel voltage Vpix is held until the scan line voltage Vg attains the high level again and the transistor (Qn) 904 is selected. The TN liquid crystal is switched in accordance with the held pixel voltage Vpix, and the light transmitted through the liquid crystal shifts from the dark state to the bright state as indicated by optical transmittance T1 in FIG. 12.
In the holding period at this time, the pixel voltage Vpix fluctuates by ΔV1, ΔV2, ΔV3 in each field, as illustrated in FIG. 12. This is caused by the fact that the capacitance of the liquid crystal varies in accordance with the response of the liquid crystal. The storage capacitor 906 usually is designed to have a large value that is two, three or more times greater than the pixel capacitance Cpix so as to make this fluctuation as small as possible. The TN liquid crystal can be driven by the pixel circuit shown in FIG. 10 by adopting the arrangement described above.
A technique for modulating the common voltage [common electrode voltage (or opposing electrode voltage)], which is illustrated in Japanese Patent Kohyo Publication No. JP-P2001-506376A (Patent Document 1), is an example of a technique having an effect that is the result of mixing the overdrive method and reset method. FIG. 2C of this reference is cited as FIG. 14 in the drawings accompanying this application.
According to the technique of Patent Document 1, ordinarily a common voltage, which is the voltage of a common electrode placed opposite a pixel electrode, is modulated. In FIG. 14, VCG indicates a temporal change in the common voltage (VCG), and an underlying waveform I indicates a temporal change in optical transmittance ascribable to response of the liquid crystal. That is, a voltage waveform 151 is a voltage waveform that is applied to the common electrode, and a light-intensity waveform 152 is a light-intensity waveform corresponding to time and conforming to the waveform 151. Reference numerals 153 to 156 denote curves of pixel light intensity.
With the prior art that preceded Patent Document 1 cited above, drive was performed with the common voltage held at a constant value [where t0 to t2 (and t2 to t4) in FIG. 14 serves as the period of one frame], or common inversion drive, in which the voltage value is varied between two voltage values at a fixed interval.
According to Patent Document 1, one frame period is divided into two parts and voltage having an amplitude substantially the same as that of conventional common inversion drive is applied in the interval from t1 to t2 (and from t3 to t4).
In the interval from t0 to t1 (and from t2 to t3) in one frame period, on the other hand, a voltage higher than the amplitude of common inversion (e.g., a voltage that is higher than the amplitude of common inversion by an amount equivalent to the voltage at the time of the black image) is applied. According to this technique, the entire display area can be changed to the black image rapidly owing to the effect of an enlarged voltage difference between the pixel electrode and common electrode in the interval from t0 to t1 over which the high voltage is applied to the common electrode. In other words, drive equivalent to reset drive is carried out.
Furthermore, even if image data is written into the pixel electrode during the interval from t0 to t1, the potential difference between the pixel electrode and common electrode is sufficiently large (e.g., greater than the black image voltage) and therefore nothing is observed on the display.
After the writing of image data to the entire display area ends, the voltage of the common electrode is returned to the amplitude of common inversion. As a result, the liquid crystal layer starts responding to change the transmittance, which conforms to each gray level, in accordance with the voltage memorized by the pixel electrode. That is, when response starts, there is a change from the state of high voltage difference to a voltage difference that conforms to each gray-le v el voltage value. In this sense a kind of overdrive is performed in the interval from t0 to t1.
Note that the response time of liquid crystal is given by the following two equations (1) and (2) (see “Liquid Crystal Dictionary”, Japan Society for the Promotion of Science, Organic Materials for Information Science, 142nd Committee, Sectional Meeting on Liquid Crystal, Baifu K. K., p. 24) (Non-Patent Document 2). Specifically, rise response (ON-time response) τrise at which a voltage higher than a threshold-value voltage is applied and the ON state attained is given by Equation (1) below.
                              τ          rise                =                                            d              2                        ·                          η              ~                                            Δɛ            ·                          (                                                V                  2                                -                                  V                  c                  2                                            )                                                          (        1        )            
On the other hand, decay response (OFF-time response) τdecay at which the applied voltage greater than the threshold value returns rapidly to zero is given by Equation (2) below.
                              τ          decay                =                                            d              2                        ·                          η              ~                                                          π              2                        ·                          K              ~                                                          (        2        )            
In Equations (1) and (2) above, d represents the thickness of the liquid crystal layer, η the rotational viscosity, Δε the dielectric anisotropy, V the applied voltage conforming to each gray level, Vc the threshold voltage, and K({tilde over ( )}) a constant based upon a Frank elastic constant. In the TN mode, the constant K is given by Equation (3) below.
                              K          ~                =                              K            11                    +                                    1              4                        ⁢                          (                                                K                  33                                -                                  2                  ·                                      K                    22                                                              )                                                          (        3        )            
In Equation (3) above, K11, K22 and K33 represent elastic constants of splay, twist and bend, respectively.
With the rise response (ON-time response), the response time of the liquid crystal depends upon the reciprocal of the square of the value of the voltage applied, as will be understood from Equation (1). In other words, the response time of the liquid crystal depends upon the reciprocal of the square in accordance with a voltage value that differs for every gray level. Depending upon the gray level, therefore, response time differs widely, and if there is a voltage difference that is ten times larger, then the difference in response time will be 100 times larger.
On the other hand, in accordance with Equation (2), a disparity in response time ascribable to the gray level exists even with the decay response (OFF-time response) but the disparity falls within the range of a two-fold increase.
Turning to Non-Patent Document 2, a higher speed is achieved owing to the overdrive effect of applying a very high voltage at the time of the rise response (ON-time response).
Further, since the response used in actual image display becomes the entire decay response (OFF-time response), dependence upon the gray level is very small. As a result, a substantially equal response time is obtained over all gray levels.
[Patent Document 1]
JP Patent Kohyo Publication No. JP-P2001-506376A
[Patent Document 2]
JP Patent No. 3039506
[Non-Patent Document 1]
H. Nakamura, K. Miwa and K. Sueoka, “Modified drive method for OCB LCD”, 1997 IDRC (International Display Research Conference), SID L-66-L-69
[Non-Patent Document 2]
“Liquid Crystal Dictionary”, Japan Society for the Promotion of Science, Organic Materials for Information Science, 142nd Committee, Sectional Meeting on Liquid Crystal, Baifukan Co., LTD, p. 24
[Non-Patent Document 3]
Tarumi et al., “Molecular Crystals and Liquid Crystals”, vol. 263, pp. 459 to 467 (Mol. Cryst. Liq. Cryst. 1995, Vol. 263, pp. 459-467